The present invention provides bone and bone tissue supplemented with at least a therapeutically useful compound and in particular relates to a method for supplementing bone and or bone tissue with said compound(s).
Major allograft surgery has provided a solution to many reconstructive problems in musculoskeletal and maxillofacial surgery. The use of such surgery remains however, retarded by the frequency of infections that are often a disabling complication of such surgery. While the use of small frozen allografts has a very low rate of infection, major allografts have infection rates between 5 and 13%. This susceptibility to infection is probably multifactorial, with avascularity and antigenicity of the implanted graft contributing as well as the frequent extensive soft tissue excision, and potential for wound breakdown.
The use of allograft bone in orthopaedic practice is now well established both as morsellised and site specific structural grafts. The risk of infection, however remains a major complicating factor with such surgery. Like other forms of allograft surgery, the frequency of infection with allograft bone varies between 5 and 13.3%. The outcome in patients who develop infection is poor and often requires either two stage revision or amputation.
Infections typically arise early after allograft surgery with 75% of cases presenting within 4 months. Perioperative introduction of organisms is the presumptive mode of infection in the majority of these cases. The most common organisms isolated are gram positive (54%) followed by gram negative (36%) and mixed (10%).
Numerous attempts have been made to lessen the rate of infection in allograft surgery, particularly in the field of maxillofacial surgery. Perioperative antibiotic regimes are often employed, involving prolonged administration of antibiotics for up to 3 months, although no controlled studies have been performed to show the efficacy of these regimes. The theoretical problem of systemic antibiotic administration in allograft surgery, particularly when using allograft bone is that the allografts are initially avascular and the antibiotics do not reach their target.
Attempts have been made to load allograft bone with antibiotics. In one such study morsellised graft was mixed with antibiotic solutions. More recently antibiotic supplemented bone allograft has been developed and used in the area of avulsive defects of the oral and maxillofacial skeleton. This technique employs demineralised particulate allograft bone and mixes it with purified gelatine powder and cephalothin and tobramycin. A canine model to test this preparation has shown a probable protection from post-operative infection when compared with conventional allografts.
Although these methods have been shown to display a decreased complication rate, the problem of infection in major allograft bone surgery is still a major concern. Furthermore, present methods of preparing allograft bone against infection require a large amount of preparatory work, are typically unsuitable where large bone grafts are required and depending on the methods used may not result in a product that has the same structural integrity as allograft bone. Thus, the problem of infection in major allograft bone surgery is largely unsolved and has severe consequences to patients who develop complications.
The present invention seeks to provide an improved bone and or bone tissue supplemented with at least a therapeutically useful compound. Moreover, the invention seeks to provide a simple and effective procedure for supplementing bone and or bone tissue with at least a therapeutically effective compound.
Throughout the specification, unless the context requires otherwise, the word xe2x80x9ccomprisexe2x80x9d or variations such as xe2x80x9ccomprisesxe2x80x9d or xe2x80x9ccomprisingxe2x80x9d, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
For the purposes of the present invention the phrase xe2x80x9cbone and bone tissuexe2x80x9d encompasses bone substitutes which comprise any biological or synthetic material used to substitute for bone during reconstruction including material processed from xenograft sources and chemicals manufactured for bone substitute purposes such as calcium phosphate and hydroxyappatite.
The present invention consists in a bone or bone tissue supplemented with at least a therapeutically useful compound, wherein said compound is concentrated within the bone matrix.
Unlike prior art products the present invention does not rely upon the use of binders, protective agents, gelatinisation agents or the like to associate therapeutic compounds with allograft bone or tissue. Rather, therapeutically effective compounds are delivered to and concentrated with the bone matrix by a process of iontophoresis. Preferably the concentration of the therapeutically effective compound within the bone is greater than the amount of therapeutically effective compound that might be absorbed into bone as a result of simple diffusion. It will be appreciated that the relative amount of therapeutic compound which might be loaded into any particular piece of bone will depend on (a) the safe in situ usage limits for that therapeutic compound, (b) the characteristics of the bone or bone tissue, (c) the biochemical characteristics of the particular compound selected and (d) the particular purpose for which the bone or bone tissue is being used.
Therapeutically effective compounds that might be employed in the invention include, but are not limited to: antibiotics, antifungal compounds and chemotherapeutic compounds, tissue growth factors (for example bone morphogenic protein), non-steroidal anti-inflammatory agents, such as indomethacin, neuromuscular agents affecting calcium and bone metabolism (such as calcitonin), anti-viral agents, anti-tuberculosis agents (such as rifampicin), anthelmintic agents (such as mebendazole), antiseptic agents, vitamins and minerals. Most preferably, the compounds that are loaded into the bone are compounds that form a salt in solution and ionise to a single positive or negative ion. Those of ordinary skill in the art will know such compounds.
If, for example, antibiotics are to be loaded into the bone or bone tissue the antibiotic compound is preferably selected from the following: flucloxacillin, gentamicin, cephalothin, ticarcillin, ciprofloxacin, nenzl-peniccillin, cefoperazone, cefuroxime, cephazolin and tobramycin. Most preferably the antibiotic is either flucloxacillin or gentamicin. When loaded into bone these compounds are preferably present at a concentration of between the minimum inhibitory concentration of the antibiotic and the concentration that would provide a total amount of antibiotic equal to the safe maximum single dose for systemic administration. For example, the maximum dose of gentamicin that might be loaded into allograft bone is about 200 mg/kg while the maximum dose of flucloxacillin is about 80 mg/kg.
If the therapeutically effective compound is an antifungal compound, the antifungal compound is preferably selected from the following: miconazole, and ketaconazole. When loaded into bone, these compounds are preferably present at a concentration of between the minimum inhibitory concentration of the antifungal and the concentration that would provide a total amount of antifungal equal to the safe maximum single dose for systemic administration.
If the therapeutically effective compound is a chemotherapeutic compound the chemotherapeutic is preferably selected from the following: 5-fluoro-uracil and vinblastin. Most preferably the chemotherapeutic is 5-fluoro-uracil.
In an alternative form, the present invention consists of a method for supplementing bone or bone tissue with a therapeutically effective compound, wherein said method employs the steps of:
(i) Exposing bone or bone tissue to a therapeutically effective compound; and
(ii) Applying a potential difference across said bone or bone tissue such that the therapeutically effective compound is concentrated within the bone or bone tissue.
Preferably, the therapeutically effective compound employed in the method is concentrated within the bone or bone tissue using an externally applied potential difference. Any externally applied potential difference may be used in the method, provided that it does not destroy the structural integrity of the bone or bone tissue. The potential difference that is used will depend on: (a) the thickness of the bone, (b) the time available to deliver the compound to the bone, (c) the compound which is to be loaded into the bone and (d) the temperature of the bone. Preferably the temperature of the bone during the loading process should be maintained below about 37xc2x0 C.
If highly externally applied potential differences are being used to load the bone with a therapeutically effective compound, then the method should be carried out in the presence of a means which is capable of cooling the bone or bone tissue. For example the method might be carried out in a refrigerated environment or alternatively might be carried out in a water bath.
It will be appreciated that the present invention is not limited to the loading of sectioned allograft bone. It might also be used in situ to deliver compounds into bone to treat medical disorders such as bone tumours. In such circumstances the therapeutically effective compound is preferable introduced at medically safe levels into the tissue surrounding the bone. An externally applied potential difference is then applied across the bone for sufficient time to concentrate the therapeutically effective compound within the bone. Preferably, the externally applied potential difference is selected such that it is capable of drawing and concentrating the therapeutically effective compound into the bone but does not effect the structural integrity of the surrounding tissue.
Therapeutically effective compounds suitable for use in the method are those which are capable of forming a soluble salt in solution. Preferably the compounds selected are capable of ionising in the presence of an externally applied potential difference to form either positive or negative ions. Examples of suitable compounds are described above. Most preferably antibiotics such as flucloxacillin and gentamicin are used as the therapeutically effective compounds.
The concentration of therapeutically effective compounds that may be loaded into the bone or bone tissue will depend largely on the properties of the compound used and the time over which the compound is required to have a therapeutic effect. Preferably the compound is concentrated within the bone to a level which exceeds the amount of compound that might be diffused into the bone as a result of diffusion over an equivalent period of time. That is, when both the present method and a diffusion method are carried out over an equivalent period of time.
Applying an external potential difference across bone or bone tissue requires the use of at least two electrodes, one being located on one side of the bone and the other being suitably positioned on the other side of the bone. To ensure electrical contact between the electrodes they are each preferably surrounded by a medium capable of conducting electrical current. Preferably there is a plurality of electrodes on either side of the bone. While any electrode might be used in the method, the preferred electrodes are those that do not produce a chemical residue that would damage the bone or bone tissue. Suitable electrodes for use in the invention include, but are not limited to, carbon, platinum, titanium, gold, noble metals, stainless steel, conductive plastic and the like.
In a highly preferred form of the invention the method is applied to allograft bone to load the bone with suitable therapeutic compounds prior to or during allograft surgery. Preferably the method is performed under aseptic conditions.
According to a particularly preferred form of the method, the section of bone to be treated is prepared and defrosted. It is then cut (in a manner which would be well known to those in skilled in the art) to an appropriate length with a slight excess at each end. Preferable that excess is in the order of about 1 to 10 mm.
To one end of the bone, a disc of an appropriate size to completely seal the medullary canal at that end is sealingly engaged to the bone. The disc can be made of any insulating material, such as acrylic, plastic etc. Sealing engagement between the bone and the disc may be achieved using, for example, a glue which is capable of bonding the disc to the bone, such as cyanoacrylate, and which is capable of being sterilised. Those of ordinary skill in the art will know such glues.
To the opposite end of the bone, a tube of an appropriate length is cemented to the bone, such that the tube sealingly engages onto the end the medullary canal (see FIG. 1). The tube can be made of any insulating material, such as acrylic. The specimen is then placed in a beaker and immersed in a buffer solution such that the open end of the extended medullary canal is just above the level of the ionic solution. The buffer solution can be any solution capable of conducting electrical current, such as normal saline.
The medullary canal is then filled with an ionic solution of the compound to be loaded into the bone. Desirably the solution is sterile and consists of an antibiotic in ionic form such as gentamicin or flucloxacillin, but may also consist of antifungal compounds, chemotherapeutic agents or any other compound that ionises to a single ionic species in solution. The pH of solution should then be optimised to ensure maximal ionisation of compound.
Electrodes are then placed in the apparatus, with at least one placed vertically in the ionic solution, and a plurality of vertical, equally spaced electrodes fixed to the side of the beaker, immersed in the buffer solution. The surrounding electrodes should then be connected electrically such that they act as one electrode. The electrodes can be made of any inert material such as carbon, or platinum.
A potential difference is then applied across the electrodes until loading of the therapeutically effective compound into the bone is complete. The time required for the compound to be transferred in to the bone tissue will depend on (a) the voltagexe2x80x94with increasing voltage the shorter the time period is required for transfer of the compound, (b) the thickness of the bone (c) the ionic compound used. Typically the maximum voltage that might be used in the method without the assistance of a suitable cooling means would be in the order of 100 V, although there is theoretically no upper or lower limit to the voltage that can be used when a cooling means is employed. Desirably the temperature of the bone should not reach or exceed 37xc2x0 C., at which temperature the collagen component of bone begins to degrade.
Voltages and times for application may be determined experimentally as described in the following examples.